What is the Bisync Protocol?

Bisync is an abbreviation shortened from
"binary synchronous". Sometimes
you may also see the acronym BSC. Bisync
is a block-oriented, error-correcting, synchronous
data communications protocol introduced by
IBM back in 1964 with the introduction of
a product called the 270X Transmission Control
Unit.

Furthermore, over the years, other terms
have come into use and are often used interchangeably
with bisync and BSC. For example, "3780
protocol", "3780 bisync",
"2780 protocol", "2780 bisync",
and "2780/3780 protocol". While
technically these terms may be inaccurate,
they do have a practical basis and convey
meaning.

To be precise, 2780 and 3780 were model
numbers
of IBM remote job entry (RJE) data
terminals
-- namely the IBM 2780 Data Communications Terminal and the IBM 3780 Data Communications Terminal . These terminals used punch cards and consisted
of a card reader, a card punch, and a line
printer. They used the bisync protocol to
transmit and receive data with an IBM mainframe
computer. Usually dial-up or leased telephone
lines and 2400 bps Bell 201C modems and then
later 4800 bps Bell 208B modems were used
to connect the terminal to the mainframe.

RJE was how programs, often referred to as
jobs, were submitted to be run on mainframe computers
back in the 1960 and 1970s. That was the
era of keypunch machines and punched cards.
The statements for a computer program, usually
COBOL or FORTRAN, and the input data for
the programs were punched onto cards using
a keypunch machine. The resulting card deck
was carried, often wheeled over on carts,
to an RJE terminal, placed into a card reader
hopper, a button was pushed, and the card
images were transmitted to the mainframe.

An immediate response may have come
back
to printer, exchange, or card punch
devices
of the terminal. This output might
have been
the result of program just submitted,
or
from some previous run. Very frequently,
the program and data were held at the
mainframe
for execution at a later time. This
is known
as batch processing.

The 3780 terminal was a later model than
the 2780 terminal and used a more robust
version of the bisync protocol -- hence the
terms "3780 bisync" vs. "2780
bisync". Virtually all bisync in use
today conforms with the 3780 version.

While it is true that "real" IBM
3780 and 2780 terminals are not in use today,
the underlying bisync protocol became the
defacto standard file transfer protocol for
a wide array of devices in the days before
the PC revolutionized computing. Much like
Zmodem and FTP today, if you needed to get
a file from one machine to another during
that time, very often bisync was protocol
used.

This massive array of hardware is not about
to disappear overnight. There is still a
huge installed base of bisync-equipped machinery
in North America and to a lesser extent in
the rest of the world.

If somehow all of the bisync interconnected
machines in the world were to disappear all
at once, the results would be catastrophic.
Many banks would cease to function. Some
air traffic control systems would collapse.
Many of the point-of-sale systems in retail
stores would fail. Many credit and debit
cards would become useless. EDI (electronic
data interchange) networks that manage much
of the business-to-business commerce would
crash. There is no doubt that bisync is still
a vital link in the chain of the world's
computer infrastructure.

Bisync is dying away. Slowly. IBM forsook
bisync years ago in favor of its SNA (Systems
Network Architecture) connectivity model.
(This led directly to the IBM 3770 data terminal family which was a successor to the 3780
-- a diskette drive was added and bisync
was replaced with a new protocol compatible
with SNA.) This abandonment, however, did
not erase the bisync protocol nor its huge
installed base. But it did open the door
for specialty companies, like Serengeti Systems, to expand into this niche market.

To some, it is remarkable that any computer-related
technology first used nearly forty years
ago could still be in use today. New equipment
relying solely on bisync connectivity probably
has not been manufactured in more than a
decade. Like the buggy whip, one day bisync
will pass into history. But, no one is holding
their breath just yet...

The following IBM publications may be helpful
if you need to explore BSC/RJE communications
in more depth:

Another good source was an article entitled
"IBM Binary Synchronous Communications
(BSC)" published by DataPro in July 1984, identified C07-491-301 Standards. (We have not been able to determine if
this article is still in print.)